Black Hole: Gravity's last laugh!

A black hole ‒ one of Einstein's general relativity's weird predictions ‒ is literally an unfathomable hole drilled in the superstructure of the Universe. Inside this hole, the immense strength of gravity distorts the structure of space and time so severely that light rays can no longer travel in a straight line, but rather follow a trajectory curving back towards the hole.
If a star has a mass greater than 3 solar mass, there is no force in the Universe that can prevent gravity from crushing it into zero volume and infinite density, known as singularity. Astronomers and physicists, used to working with objects of finite size and density, are ill at ease with singularities. Nature is also "embarrassed" by the collapse of a massive object into a point. Hence, it conceals the object from us inside a region of space called "black hole."
Although black holes confound common sense, they are real albeit oddballs in the stellar "zoo." Super massive black holes at the center of many galaxies have non-stellar origin. These gravitational Frankensteins are due to the collapse of interstellar matter with mass equal to millions and billions of solar masses.
A black hole is separated from the rest of the Universe by an imaginary boundary called the event horizon. It is a bizarre terminus beyond which everything is forever hidden from our view. Once a massive dying star collapses to within its event horizon, it disappears permanently from the Universe, leaving behind black hole as relic of its past life
However, because of time-dilation, an off-shoot of Einstein's special relativity, time slows to a crawl at the event horizon, and the process of falling into the black hole appears to take forever. According to astrophysicist John Wheeler, black hole "teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blow-out flame, and that the laws of physics that we regard as 'sacred,' as immutable, are anything but."
The distance from the center of the black hole to the event horizon is called the Schwarzschild radius, named after a German physicist who in 1916 advanced the idea of black holes. It is 3 kilometer per solar mass. Thus if a 3 solar mass star shrinks to a radius of 9 kilometer, it will become a black hole; for a 4 solar mass star the radius is 12 kilometer, and so on. Our Sun will never become a black hole because it is not massive enough to cross over the event horizon. Instead its luminous career will end more sedately as a white dwarf.
A black hole is well nigh impossible to detect because light from inside the hole cannot escape to the region beyond the event horizon. Hence, there is a total information blackout. How do we then infer about its existence?
In many binary systems, two stars orbiting about their common center of mass, it is often found that one of the orbiting stars is invisible. The invisible companion, betraying its presence by exerting gravitational pull, draws in material from its companion's atmosphere. As the material falls into the hole, it emits blazing X-rays. This is a signature that the invisible star is a black hole. Many such black holes have been discovered in our galaxy, the Milky Way, by the Hubble telescope.
Study of rotational motion of galaxies allows us to figure out the mass at the galactic center. For some galaxies, including ours and our nearest neighbor Andromeda, the mass is found to be millions and billion times the solar mass. Concentration of so much mass in so small a region is possible only in super massive black holes
The space-based Chandra X-ray Observatory, named after the Nobel astrophysicist Subramanyan Chandrashekhar and launched by NASA in 1999, has been specially designed to detect black holes. Since its launching, Chandra's success has been phenomenal. To date, it detected numerous black holes, including super massive ones at the center of galaxies as far away as 2.7 billion light years from Earth. Chandra is not only unveiling black holes all over the cosmos, it is also challenging our prevailing ideas about how they are formed and providing a fertile ground for theoretical speculations.
Black holes are proof that gravity controls the life of stars and has the last laugh. It is a star's one-way ticket to eternity.

The writer is a Professor of Physics at Fordham University, New York.